The present invention relates to a method and an apparatus for removing liquid from materials such as suspensions or slurries, especially by the application of high pressure, so as to produce material is relatively dry.
Several different forms of apparatus, often called web presses, have been known in which a suspension is carried between a pair of driven webs and liquid is removed from the suspension by a squeezing pressure applied by the webs. By passing the webs, which are pervious to liquid, partly around at least one press drum or roll while under tension the suspension carried between them is subjected to a squeezing pressure, thereby removing liquid from the suspension. In order to increase the degree of removal of liquid, a further pressure, in addition to that caused by the web tension, may be applied by a pressure belt or other device which forces the outer web toward the surface of the press drum.
In many such web presses the webs enclose a substantial part of the circumference of the press drum, often exceeding an angle of 180.degree., and the additional pressure is applied to the outer web along almost the entire region of contact of the webs with the drum surface, resulting in a lateral load on the press drum. This lateral load causes problems with both the press drum and its bearings, especially when a high additional pressure is applied in order to achieve the greatest possible removal of liquid from the suspension. To overcome the problems caused by the lateral load on the press drum it has been necessary to make both the press drum and its bearings larger and stronger than would be required to withstand the pressure applied to the suspension. This is a disadvantage for many reasons, including the increased cost and weight of the belt press.
Furthermore, it has been shown that, at least in certain cases, when continuous pressure is applied to a suspension carried between two webs the desired degree of liquid removal is only achieved during the first portion of the pressure application and the continued application of the same pressure is relatively ineffective in accomplishing liquid removal. Moreover some web presses have webs which are up to two meters or more in width and require correspondingly long press drums. With such long press drums, however, the application of additional pressure in the manner described above causes a significant and undesirable deflection or bending of the press drum.
Accordingly, it is an object of the present invention to avoid the necessity for making the press drum and its bearings larger or stronger so as to withstand the application of additional pressure to a suspension carried between a pair of webs.
Another object of this invention is to make possible the application of additional pressure to a long press drum without risk of deflection of the drum while achieving at least as great a degree of liquid removal as in previously known web presses.
A further object of the invention is to achieve effective liquid removal during the last part as well as the first part of a continuous passage of a pair of webs around a press drum.